Try www.hawkbullets.com. Hawk bullets has spitzers from 300-450 grains. You can get them with different jacket thicknesses to control how fast or slow the expansion. At least that's how I read it on their website. I don't have any experience with them personally but I have never heard anything bad about them (other than being a little expensive).

You're not going to find much because the .45 caliber rifle bullet is designed either for tubular magazines or for dangerous game, most of which is taken at fairly short range. In both cases it has been proven that a HEAVY round or flat nosed bullet of very solid construction is best.

The 45/70 is not a long range cartridge. Yes, the buffalo hunters used it at very long range but they also had EXTREMELY long barrels and trajectories that put the bullet into orbit before it hit the target.

I wish you luck on your quest but I think you are asking something that the round is not capable of. Hopefully somebody will know something that I don't. Let us know what you find. I'm sure there are others in the same boat.